**Core Concept**
Activated protein C (APC) is a serine protease that plays a crucial role in regulating coagulation and inflammation. It exerts its effects by inactivating factors Va and VIIIa, thereby inhibiting the coagulation cascade.

**Why the Correct Answer is Right**
Activated protein C is used therapeutically to treat conditions characterized by excessive coagulation and thrombosis. It acts by activating protein S, a cofactor that enhances the inactivation of factors Va and VIIIa. This mechanism is particularly beneficial in conditions such as sepsis-induced coagulopathy and disseminated intravascular coagulation (DIC). The therapeutic use of APC is also indicated in patients with severe pulmonary embolism and in those at high risk of recurrent venous thrombosis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because heparin is an anticoagulant that works by activating antithrombin, which inhibits thrombin and factor Xa.
**Option B:** This option is incorrect because aspirin is an antiplatelet agent that inhibits platelet aggregation by irreversibly inhibiting cyclooxygenase-1 (COX-1).
**Option C:** This option is incorrect because tissue plasminogen activator (tPA) is a thrombolytic agent that works by activating plasminogen to plasmin, which then degrades fibrin clots.

**Clinical Pearl / High-Yield Fact**
Activated protein C has a narrow therapeutic window, and its use is contraindicated in patients with a history of heparin-induced thrombocytopenia or those receiving anticoagulant therapy with heparin or warfarin.

**Correct Answer: D. Severe pulmonary embolism**